The present invention concerns a novel system for cardiac pacing and sensing.
In both unipolar and bipolar cardiac pacing, in which a lead is located in a cardiac chamber, after a current pulse is applied polarization charges exist in solution at the electrode-electrolyte interface. These charges are electrolytic ions which are intrinsic to the volume conductor. Anions are found surrounding the anode while cations are established at the cathode. As a result, during sensing it is often difficult to sense the evoked cardiac response because the polarization voltages may be greater than the cardiac signals. In effect, the sensing circuit of the pacer is unable to determine whether the pulse provided by the pacer has been an effective stimulus because the pulse that was provided results in a polarization charge that masks the cardiac response. It has been determined to be the pacer's sensing circuit that is effectively unable to determine the response.
The use of a three-electrode configuration in the atrium for the purpose of reducing cross-sensing (i.e., cross-talk) from the ventricle during sensing has been suggested prior to this invention. I have discovered that by using three electrodes in either of the cardiac chambers, all three electrodes may be used for sensing and two of the same three electrodes may be used for pacing with the result that in addition to alleviating the cross-talk problem as previously noted, the polarization problem is also alleviated. I have found that by using the three electrodes, the polarization potential as seen by the electrodes is reduced sufficiently so that the electrodes readily can "read" the cardiac evoked response.
Other objects and advantages of the present invention will become apparent as the description proceeds.